There are many ways to create this type of effect, for me this is the easiest and fastest one, 4 HD Photoshop Brushes that you can adjust and edit as you wish, I use them a lot in my backgrounds, sometimes they're very subtle, other times they're an important part of the composition, I paint them on a separate layer so I can color them or change the blending mode to create all kinds of different effects... I got them from a friend many years ago, and now I'm passing them to you, use them wisely 😄